1. Field of the Invention
This invention relates to a process for reclaiming vulcanized rubber and to the resultant mixture of organic compounds.
2. Discussion of the Background
In recent years the problem of disposing of discarded tires has become acute. At present over 200,000,000 tires are discarded annually and much research has been directed to addressing this problem.
Scrap tires have been disposed of by open-burning. However, this results in an intolerable level of air pollution. Non-polluting incineration of scrap tires is possible, yet, this method is viewed only as a matter of expediency and is wasteful of a rather valuable raw material. Incineration with waste-heat recovery to generate steam is another method to utilize scrap tires. However, this procedure is not a true recycling technology and again is wasteful of a valuable resource.
Disposal of scrap tires without reclamation of some, if not most, of the material therein must be considered extremely wasteful. Accordingly, efforts have been directed towards recovery of this potential resource.
Few practical methods have been developed for recycling scrap tires. Reclaiming of scrap rubber has been practiced for many years, with digestion and separation of textile materials and some "de-vulcanization" and degradation of the polymer. Recent increases in processing costs have made these operations uneconomical, however, and many plants have discontinued operation. Thus, the need exists for a reclaiming process which would permit inexpensive recycling of scrap tires to recover the organic material therein. Pyrolysis, either in bulk or in solution, has also been explored to degrade and recover the styrene-butadiene copolymer in scrap tires, see Larsen et al, Rubber Chem. Technol., 49, 1120 (1976), Lucchesi et al, Conservation & Recycling, 6, 85 (1983), Kawakami et al, ACS Symp. Ser. 130, 557 (1980).
However, compared to the studies on the thermal depolymerization of polystyrene, polybutadienes and their blends, the research on pyrolytic recovery of styrene-butadiene copolymer has not been fully developed. The degradation reaction of styrene-butadiene rubber in tetralin at 140.degree. C. was examined but the resulting material was not otherwise characterized, except to note it as randomly cross-linked material, see Gur et al, Indian J. Chem., 6, 495 (1969).
High-boiling naphthenic oils have also been utilized to dissolve scrap rubber, suggesting that the hot oil serves both physically as a solvent and a heat-transfer medium promoting chain scission, and chemically as a chain-transfer vehicle, see Crane et al, Rubber Chem. Technol., 48, 50 (1975). Although the resulting product was asserted to be a low molecular-weight depolymerized material, no attempts were made to investigate the degradation components.
The use of a supercritical fluid (SCF) as a reaction medium can provide an alternative approach for lowering the operating temperature of pyrolysis reactions. Improved yields and selectivities have been reported in an SCF reaction medium when compared with the results obtained under pyrolysis, see McHugh et al, Supercritical Fluid Extraction-Principles and Practice; Butterworth; Stoneham, pp. 195-215 (1986). Supercritical acetone has been used as the reaction medium for the thermal degradation of cellulose, obtaining higher extraction yields at temperatures lower than those used for conventional pyrolysis, see Koll et al, Angew. Chem. Int. Ed. Engl., 17, 754 (1986). Thermal intermolecular organic reactions have been studied in supercritical fluid media at pressures of up to 50.7 MPa and temperatures of up to 500.degree. C. and found that alkanes could be coupled ito alkenes, to 1,3-dienes, and to alkynes, see Metzger et al, Chemical Engineering at Supercritical Conditions; Paulaitis et al, Eds.; Ann Arbor Science; Ann Arbor, Mich. (1983); Chapter 26. The oxidation of n-butane in both liquid and SCF phases has also been studied, see, The Oxidation of Hydrocarbons in the Liquid Phase; Emanuel, N. M., Ed.; McMillan; New York, 1985.
A unique feature of a supercritical fluid is that it displays a wide spectrum of solvent characteristics. To a first approximation, the solvent power of a supercritical fluid can be related to the solvent density in the critical region. For a reduced temperature (T.sub.R) range of 0.9 to 1.2 and at reduced pressures (P.sub.R) greater than 1.0, the reduced density (.rho..sub.R) of the solvent can vary from a value of approximately 0.1 to 0.9, which possess gas-like densities, and a range of 1.0 to approximately 2.5, which possess liquid-like densities. When operating in the critical region, both pressure and temperature can be used to regulate the density and solvent power of a supercritical fluid.
The extraction of organic material from used tires with supercritical fluids has been reported, see Funazukuri et al, Journal of Chemical Engineering of Japan, 18, 455 (1985), however, such an extraction employed supercritical solvents whose densities were in the gas-like range. As such, the extraction is time-consuming and the recovery of organic material is incomplete. The average molecular weight of the resultant organic material ranges from 325 to 480 daltons. Accordingly, this reference fails to recognize the advantages which are obtained from the production of a valuable mixture of organic compounds from the reclamation process. This reference neglects to adjust the parameters of the extraction process so as to obtain a commercially important material. Rather the process of this reference merely converts one form of waste (scrap tires) to a second almost equivalently useless form.